This invention relates generally to curable, weldable coatings for metal substrates, and more particularly, to curable, weldable coatings for metal substrates, which inhibit corrosion.
The production of light gauge steel for end uses ranging from architectural construction materials to automobiles is well known. A rolling mill line produces continuous sheets of steel in the required thickness and width. The steel sheets may be coated with a thin layer of zinc metal via a galvanizing process. Eventually, mill oil is applied to the uncoated or galvanized steel sheets, and the steel is either stored or shipped in a coil to a customer for further processing.
Typically, the customer is an automobile manufacturer who will take the coiled metal sheet and pass it through a lubricating station and then to a forming operation where the metal sheet is cut and formed into a part such as a roof, fender, door, etc. The parts are then welded together to form an automobile body. Next, the automobile body is cleaned, treated with a zinc phosphating solution to enhance corrosion protection, and rinsed with deionized water. The treated automobile body is then passed through an electrodeposition bath where a corrosion resistant primer is applied.
The automobile manufacturers would like to streamline their operations and have some of the operations described above done outside the automobile assembly plant, for example at a steel mill or a custom coater. One major problem with moving certain operations to a steel mill or a custom coater is that any coating applied outside the automobile assembly plant must be able to accept a weld. At some point in time, the various metal parts will be welded together in the automobile assembly plant to form the automobile body. Consequently, automobile manufacturers have a strong demand for a weldable, corrosion resistant coating composition that can be applied at a steel mill or at a custom coating facility.
Such a weldable, corrosion resistant coating composition could be applied at a custom coater, known as a coil coater, who would ship the coated metal sheet to the automobile manufacturer. As described above, the automobile manufacturer would then form the metal sheet into parts and weld the parts together. However, the metal pretreatment operation and perhaps the electrodeposition process could be avoided since the metal received by the automobile manufacturer would already contain a corrosion resistant coating.
Similar to the above, a weldable, corrosion resistant coating composition could also be applied at a steel mill. Application at the steel mill enables the automobile manufacturer to receive corrosion resistant metal directly without the expense associated with shipping the metal to a coil coater and from the coil coater to the automobile manufacturer.
The present invention provides a weldable, curable coating composition that provides corrosion protection and can be applied by a coil coater or at a steel mill, can be cured at low temperature and provides good adhesion and good corrosion protection without prior metal pretreatment.
One aspect of the present invention is a curable coating composition comprising:
a. a resinous binder comprising:
i. a reaction product of an epoxy-containing polymer with a compound containing phosphorus acid groups, the reaction product having reactive functional groups,
ii. a curing agent having functional groups reactive with the functional groups of (i);
b. an electroconductive pigment dispersed in (a) such that the weight ratio of b to (i) plus (ii) is within the range of 0.5 to 9.0:1,
the curable coating composition being characterized such that when it is deposited and cured on a metal substrate, the cured coating is weldable.
Another aspect of the present invention is an aqueous-based coating composition comprising:
a. a resinous binder comprising:
i. a reaction product of an epoxy-containing polymer with a compound containing phosphorus acid groups, the reaction product having reactive functional groups,
ii. a curing agent having functional groups reactive with the functional groups of (i);
b. an electroconductive pigment dispersed in (a) such that the weight ratio of b to (i) plus (ii) is within the range of 0.5 to 9.0:1; and
c. water,
the coating composition being characterized such that when it is deposited and cured on a metal substrate, the cured coating is weldable.
Yet, another aspect of the present invention is an organic solvent-based coating composition comprising:
a. a resinous binder comprising:
i. a reaction product of an epoxy-containing polymer with a compound containing phosphorus acid groups, the reaction product having reactive functional groups,
ii. a curing agent having functional groups reactive with the functional groups of (i);
b. an electroconductive pigment dispersed in (a) such that the weight ratio of b to (i) plus (ii) is within the range of 0.5 to 9.0:1; and
c. an organic solvent,
the curable coating composition being characterized such that when it is deposited cured on a metal substrate, the cured coating is weldable.
Another aspect of the present invention is a process for coating a continuous strip or coil of metal comprising:
a. applying directly to the metal sheet shortly after it is formed and at a temperature of 20 to 150xc2x0 C., a curable coating composition comprising:
i. a resinous binder comprising
(A) a reaction product of an epoxy-containing polymer with a compound containing phosphorus acid groups, the reaction product having reactive functional groups,
(B) a curing agent having functional groups reactive with the functional groups of (A);
ii. an electroconductive pigment dispersed in (i) such that the weight ratio of (ii) to (A) plus (B) is within the range of 0.5 to 9.0:1,
the curable coating composition being characterized such that when it is deposited and cured on a metal substrate, the cured coating is weldable; and
b. drying the coating composition on the metal sheet.
Yet, another aspect of the invention is a process for coating a continuous metal coil comprising:
a. unwinding the metal sheet from a metal coil and passing the metal sheet in a substantially continuous manner through a cleaning station, a coating station, and a curing station;
b. applying to the metal sheet at the coating station a curable coating composition comprising:
i. a resinous binder comprising:
(A) a reaction product of an epoxy-containing polymer with a phosphorus-containing acid, the reaction product having reactive functional groups,
(B) a curing agent having functional groups reactive with the functional groups of (A);
ii. an electroconductive pigment dispersed in (i) such that the weight ratio of (ii) to (A) plus (B) is within the range of 0.5 to 9.0:1; and
C. curing the coating composition applied to the metal sheet in step (b) as the coated metal sheet passes through the curing station.
The use of numerical values in the various ranges specified in this application, unless expressly indicated otherwise, are stated as approximations as though the minimum and maximum values within the stated ranges are both preceded by the word xe2x80x9caboutxe2x80x9d. In this manner, slight variations above and below the stated ranges can be used to achieve substantially the same results as values within the ranges. Also, the disclosure of these ranges is intended as a continuous range including every value between the minimum and maximum values.
The present invention is a curable coating composition for metal substrates that can be applied without pretreatment and methods involving the same. The curable coating composition comprises a resinous binder. The resinous binder comprises a reaction product of an epoxy-containing polymer with a compound containing phosphorus acid groups. The reaction product has reactive functional groups.
Useful epoxy-containing polymers have at least one epoxy or oxirane group in the molecule, such as polyglycidyl ethers of polyhydric alcohols. Useful polyglycidyl ethers of polyhydric alcohols can be formed by reacting epihalohydrins like epibromohydrin, dichlorohydrin and epichlorohydrin with polyhydric alcohols, such as dihydric alcohols, in the presence of an alkali condensation and dehydrohalogenation catalyst. Suitable alkali condensation and dehydrohalogenation catalyst include sodium hydroxide or potassium hydroxide.
Suitable polyhydric alcohols can be aromatic, aliphatic or cycloaliphatic. Non-limiting examples of suitable aromatic polyhydric alcohols include phenols that are preferably at least dihydric phenols. Other useful aromatic polyhydric alcohols include dihydroxybenzenes, for example resorcinol, pyrocatechol and hydroquinone; bis(4-hydroxyphenyl)-1,1-isobutane; 4,4-dihydroxybenzophenone; bis(4-hydroxyphenyl)-1,1-ethane; bis(2-hydroxyphenyl)methane; 1,5-hydroxynaphthalene; 4-isopropylidene bis(2,6-dibromophenol); 1,1,2,2-tetra(p-hydroxy phenyl)-ethane; 1,1,3-tris(p-hydroxy phenyl)-propane; novolac resins; bisphenol F; long-chain bisphenols; and 2,2-bis(4-hydroxyphenyl)propane, i.e., bisphenol A.
Non-limiting examples of aliphatic polyhydric alcohols include glycols such as ethylene glycol, diethylene glycol, triethylene glycol, 1,2-propylene glycol, 1,4-butylene glycol, 2,3-butylene glycol, pentamethylene glycol, polyoxyalkylene glycol; polyols such as sorbitol, glycerol, 1,2,6-hexanetriol, erythritol and trimethylolpropane; and mixtures thereof. An example of a suitable cycloaliphatic alcohol is cyclohexanedimethanol.
Preferably, the epoxy-containing polymer has at least two epoxy groups per molecule and aromatic or cycloaliphatic functionality to improve adhesion to a metal substrate. It is also preferred that the epoxy-containing polymer be relatively more hydrophobic than hydrophilic in nature. Further, the epoxy-containing polymer should have a number average molecular weight of about 220 to 25,000. The molecular weight can be determined by multiplying the epoxy equivalent weight or epoxy equivalent by the epoxy functionality or number of epoxy groups.
Useful epoxy-containing polymers are disclosed in U.S. Pat. Nos. 5,294,265; 5,306,526 and 5,653,823, which are hereby incorporated by reference. Other useful epoxy-containing materials include epoxy-functional acrylic polymers, glycidyl esters of carboxylic acids and mixtures thereof. Examples of suitable commercially available epoxy-containing polymers are available from Shell Chemical Company under the trademarks EPON(copyright) 836, EPON(copyright) 828, EPON(copyright) 1002F and EPON(copyright) 1004F. EPON(copyright) 836 and EPON(copyright) 828 are epoxy functional polyglycidyl ethers of bisphenol A prepared from bisphenol A and epichlorohydrin. EPON(copyright) 828 has a number average molecular weight of about 400 and an epoxy equivalent weight of about 185-192. EPON(copyright) 836 has an epoxy equivalent weight of about 178-186.
The compound containing phosphorus acid groups that is reacted with the epoxy-containing polymer comprises phosphonic acids, phosphorous acid, phosphoric acids (preferred) including super- and poly-, and mixtures thereof.
Examples of suitable phosphonic acids include those having at least one group of the structure:
xe2x80x94Rxe2x80x94POxe2x80x94(OH)2
where R is xe2x80x94Cxe2x80x94, preferably CH2, and more preferably Oxe2x80x94COxe2x80x94(CH2)2xe2x80x94. Nonlimiting examples of suitable phosphonic acids include 1-hydroxyethylidene-1,1-diphosphonic acid, methylene phosphonic acids, and alpha-aminomethylene phosphonic acids containing at least one group of the structure: 
such as (2-hydroxyethyl)aminobis(methylene phosphonic) acid, isopropylaminobis(methylenephosphonic) acid and other aminomethylene phosphonic acids disclosed in U.S. Pat. No. 5,034,556 at column 2, line 52 to column 3, line 43, which is hereby incorporated by reference.
Other useful phosphonic acids include alpha-carboxymethylene phosphonic acids containing at least one group of the structure: 
Nonlimiting examples of suitable phosphonic acids include benzylaminobis(methylene phosphonic) acid, cocoaminobis(methylene phosphonic) acid, triethylsilylpropylamino(methylene phosphonic) acid and carboxyethyl phosphonic acid.
The equivalent ratio of the compound containing phosphorus acid groups to epoxy-containing polymer is within the range of 0.3 to 5.0:1, preferably 0.5 to 3.5:1. The epoxy-containing polymer and the compound-containing phosphorus acid groups can be reacted together by any method known to those skilled in the art.
The functional groups associated with the reaction product of the epoxy-containing polymer and the compound-containing phosphorus acid groups are hydroxyl groups including acidic hydroxyls or hydroxyl groups and epoxy groups depending on the equivalent ratio of the compound containing phosphorus acid groups to epoxy-containing polymer.
The resinous binder of the present invention also comprises a curing agent having functional groups that are reactive with the functional groups of the reaction product described above. The curing agent can be selected from aminoplasts, polyisocyanates, including blocked isocyanates, polyacids, organometallic acid-functional materials, polyamines, polyamides and mixtures of any of the foregoing depending on the identity of the functional groups in the reaction product.
Useful aminoplasts can be obtained from the condensation reaction of formaldehyde with an amine or amide. Nonlimiting examples of amines or amides include melamine, urea and benzoguanamine.
Although condensation products obtained from the reaction of alcohols and formaldehyde with melamine, urea or benzoguanamine are most common, condensates with other amines or amides can be used. For example, aldehyde condensates of glycoluril, which yield a high melting crystalline product useful in powder coatings, can be used. Formaldehyde is the most commonly used aldehyde, but other aldehydes such as acetaldehyde, crotonaldehyde, and benzaldehyde can also be used.
The aminoplast can contain imino and methylol groups. In certain instances, at least a portion of the methylol groups can be etherified with an alcohol to modify the cure response. Any monohydric alcohol like methanol, ethanol, n-butyl alcohol, isobutanol, and hexanol can be employed for this purpose. Nonlimiting examples of suitable aminoplast resins are commercially available from Cytec Industries, Inc. under the trademark CYMEL(copyright) and from Solutia, Inc. under the trademark RESIMENE(copyright). Preferred aminoplasts are CYMEL(copyright) 385 (preferred for water-based compositions), CYMEL(copyright) 1158 imino-functional melamine formaldehyde condensates, and CYMEL(copyright) 303.
Other curing agents suitable for use include, but are not limited to, polyisocyanate curing agents. As used herein, the term xe2x80x9cpolyisocyanatexe2x80x9d is intended to include blocked (or capped) polyisocyanates as well as unblocked polyisocyanates. The polyisocyanate can be aliphatic, aromatic, or a mixture of the foregoing. Although higher polyisocyanates such as isocyanurates of diisocyanates are often used, diisocyanates can be used. Higher polyisocyanates also can be used in combination with diisocyanates. Isocyanate prepolymers, for example reaction products of polyisocyanates with polyols also can be used. Mixtures of polyisocyanate curing agents can be used.
If the polyisocyanate is blocked or capped, any suitable aliphatic, cycloaliphatic, or aromatic alkyl monoalcohol known to those skilled in the art can be used as a capping agent for the polyisocyanate. Other suitable capping agents include oximes and lactams. Other useful curing agents comprise blocked polyisocyanate compounds such as, for example the tricarbamoyl triazine compounds described in detail in U.S. Pat. No. 5,084,541, which is incorporated herein by reference.
Suitable curing agents are described in U.S. Pat. No. 4,346,143 at column 5, lines 45-62 and include blocked or unblocked di- or polyisocyanates such as toluene diisocyanate blocked with caprolactam. A toluene diisocyanate blocked with caprolactam is commercially available from Bayer Corporation under the trademark DESMODUR(copyright) BL 1265.
Suitable polyacid curing agents include acid group-containing acrylic polymers prepared from an ethylenically unsaturated monomer containing at least one carboxylic acid group and at least one ethylenically unsaturated monomer that is free from carboxylic acid groups. Such acid functional acrylic polymers can have an acid number ranging from 30 to 150. Acid functional group-containing polyesters can be used as well. The above-described polyacid curing agents are described in further detail in U.S. Pat. No. 4,681,811 at column 6, line 45 to column 9, line 54, which is incorporated herein by reference.
Useful organometallic complexed materials which can be used as curing agents include a stabilized ammonium zirconium carbonate solution commercially available from Magnesium Elektron, Inc. under the trademark BACOTE(trademark) 20, stabilized ammonium, zirconium carbonate, and a zinc-based polymer crosslinking agent commercially available from Ultra Additives Incorporated under the trademark ZINPLEX 15.
Nonlimiting examples of suitable polyamine curing agents include primary or secondary diamines or polyamines in which the radicals attached to the nitrogen atoms can be saturated or unsaturated, aliphatic, alicyclic, aromatic, aromatic-substituted-aliphatic, aliphatic-substituted-aromatic, and heterocyclic. Nonlimiting examples of suitable aliphatic and alicyclic diamines include 1,2-ethylene diamine, 1,2-propylene diamine, 1,8-octane diamine, isophorone diamine, propane-2,2-cyclohexyl amine, and the like. Nonlimiting examples of suitable aromatic diamines include phenylene diamines and toluene diamines, for example o-phenylene diamine and p-tolylene diamine. These and other suitable polyamines are described in detail in U.S. Pat. No. 4,046,729 at column 6, line 61 to column 7, line 26, which is incorporated herein by reference.
Appropriate mixtures of curing agents may also be used in the invention. The weight percent of the curing agent generally ranges from 5 to 60 percent based on the total weight of the resinous binder.
The curable coating also comprises an electroconductive pigment dispersed in the resinous binder. Nonlimiting examples of suitable electroconductive pigments include zinc, aluminum, iron, graphite, iron phosphide, tungsten, stainless steel, and mixtures thereof. Suitable zinc pigments are commercially available from ZINCOLI GmbH under the trademark ZINCOLIS 620 or 520. Suitable iron phosphide pigments are commercially available from Occidental Chemical Corporation under the trademark FERROPHOS(trademark).
The electroconductive pigment is dispersed in the resinous binder such that the curable coating composition deposited and cured on a metal substrate is weldable. The term xe2x80x9cweldablexe2x80x9d is defined as being sufficiently electroconductive to sustain a spot welding and joining operation as used in an automotive assembly plant. Preferably, the weight ratio of the electro-conductive pigment to the reaction product plus curing agent is within the range of 0.2 to 10. Also, the weight percent of electroconductive pigment based on the total weight of resinous binder plus electroconductive pigment is from 30 to 95 percent.
The curable coating composition may contain a diluent. Diluents are added to adjust the viscosity of the coating composition. If a diluent is used, it should not detrimentally affect the adhesion of the curable coating composition to the metal substrate. Useful diluents include water, organic solvents, or mixtures of water and organic solvents.
When water is included as a diluent, dispersants, thickeners, stabilizers, rheology modifiers, and anti-settling agents are required. A suitable rheology modifier is available from Rohm and Haas Company under the trademark Rheology Modifier QR-708, Experimental. A suitable stabilizing and dispersing agent is potassium tripolyphosphate (KTPP). When prepared, the viscosity of the aqueous composition is 300-12,000 cp (Brookfield Cone and Plate). When the composition is shipped, it is up to 35 percent water by weight with a viscosity of about 100-2000 cp. At application, the composition will be no more than 50 percent water by weight with a viscosity between 20-100 cp.
Optimally, the aqueous composition will contain an amine. The preferred amines are hydroxyl-containing amines. The volatile organic compound content (VOC content) of the aqueous composition will be less than 2, preferably less than 1.7.
Method 24 is a common method for determining VOC content. According to Method 24, the VOC content for single component coatings is determined by calculating the total volatile content in grams for the water and/or exempt material content in grams and dividing by the volume of the test specimen corrected for the water and/or exempt material volume. The VOC content is reported as the mass per unit volume of coating (grams per liter or pounds per gallon) or as the mass per unit volume of coating solids (grams per liter of solids).
For multi-component coatings, the VOC content is determined using the following equations:       VOC    =                                                      (                              total                ⁢                                  xe2x80x83                                ⁢                volatiles                ⁢                                  xe2x80x83                                ⁢                less                ⁢                                  xe2x80x83                                ⁢                water                ⁢                                  xe2x80x83                                ⁢                less                ⁢                                  xe2x80x83                                ⁢                exempt                ⁢                                  xe2x80x83                                ⁢                solvents                            )                                                                          (                              density                ⁢                                  xe2x80x83                                ⁢                of                ⁢                                  xe2x80x83                                ⁢                coating                            )                                                                                                      100                ⁢                %                            -                              (                                  volume                  ⁢                                      xe2x80x83                                    ⁢                  percent                  ⁢                                      xe2x80x83                                    ⁢                  of                  ⁢                                      xe2x80x83                                    ⁢                  water                                )                            -                                                                          (                              volume                ⁢                                  xe2x80x83                                ⁢                percent                ⁢                                  xe2x80x83                                ⁢                of                ⁢                                  xe2x80x83                                ⁢                exempt                ⁢                                  xe2x80x83                                ⁢                solvents                            )                                            or      VOC    =                            (                      W            o                    )                ⁢                  (                      D            c                    )                                      100          ⁢          %                -                  V          W                -                  V          ex                    
Where:
WO=weight percent of organic volatiles
VW=volume of water, %, (WW) (DC/DW)
Vex=volume of exempt solvent, %, (Wex) (DC/Dex)
DC=density of coating, g/L, at 25xc2x0 C.
The VOC content for multi-component coatings is expressed as the mass of VOC per unit volume of the coating minus water and exempt solvents.
The diluent of the present invention can be an organic solvent. Suitable organic solvents include alcohols having up to about 8 carbon atoms, such as ethanol and isopropanol; and alkyl ethers of glycols, such as 1-methoxy-2-propanol, and monoalkyl ethers of ethylene glycol, diethylene glycol and propylene glycol. Preferably, the diluent includes a propylene glycol monomethyl ether or a dipropylene glycol monomethyl ether that are commercially available from Dow Chemical Company. A suitable propylene glycol monomethyl ether is available from Dow Chemical Company under the trademark DOWANOL PM. A suitable dipropylene glycol monomethyl ether is commercially available under the trademark DOWANOL DPM.
Other suitable organic solvents include ketones such as cyclohexanone (preferred), acetone, methyl ethyl ketone, methyl isobutyl ketone and isophorone; esters and ethers such as 2-ethoxyethyl acetate, propylene glycol methyl ether acetates such as PM ACETATE, which is commercially available from Dow Chemical Company; and aromatic solvents such as toluene, xylene, aromatic solvent blends derived from petroleum such as those available under the trademark SOLVESSO(copyright).
When prepared, the viscosity of the organic solvent-containing composition is 300-12,000 cp (Brookfield Cone and Plate). When the composition is shipped, it is 20-40 percent organic solvent by weight with a viscosity of about 100-2000 cp. At application, the composition will be approximately 50 percent organic solvent by weight, the composition with a viscosity between 20-100 cp.
The solvent-based composition contains an amine for stability purposes. The preferred amines are alkyl substituted morpholine compounds such as N-methyl and N-ethyl morpholine.
Optimally, the curable coating composition of the invention can further comprise surfactants. Surfactants can be used to improve the wetting of the substrate. Generally, surfactants are present in an amount of less than about 2 weight percent on a basis of total weight of the coating composition. Suitable surfactants are commercially available from Air Products and Chemicals, Inc. under the trademark SURFYNOL 104 PA.
The coating composition of the present invention can also include corrosion resistant pigments. Suitable corrosion resistant pigments include, but are not limited to, zinc phosphate, calcium ion-exchanged silica, colloidal silica, synthetic amorphous silica, and molybdates such as calcium molybdate, zinc molybdate, barium molybdate, strontium molybdate, and mixtures thereof. Suitable calcium ion-exchanged silica is commercially available from W. R. Grace and Co. under the trademark SHIELDEX(copyright) AC3. Suitable colloidal silica is available from Nissan Chemical Industries, Ltd. under the trademark SNOWTEX. Suitable amorphous silica is available from W. R. Grace and Co. under the trademark SYLOID(copyright).
The curable coating composition can further comprise other optional ingredients such as inorganic lubricants like molybdenum disulfide particles that are commercially available from Climax Molybdenum Marketing Corporation. The coating composition can also include extender pigments such as iron oxides and iron phosphides, flow control agents, and thixotropic agents such as silica, montmorillonite clay and hydrogenated castor oil. Further, the coating composition can include anti-settling agents such as aluminum stearate and polyethylene powder, dehydrating agents which inhibit gas formation such as silica, lime or sodium aluminum silicate, and wetting agents including salts of sulfated castor oil derivatives such as those commercially available from Cognis Corporation under the trademark RILANIT R4.
Preferably, the curable coating composition is essentially free of chromium-containing materials, i.e., contains less than about 2 weight percent of chromium-containing materials (expressed as CrO3), and more preferably less than about 0.05 weight percent of chromium-containing materials, and most preferably about 0.00001 weight percent. Examples of such chromium-containing materials include chromic acid, chromium trioxide, chromic acid anhydride, dichromate salts such as ammonium dichromate, sodium dichromate, potassium dichromate, and calcium chromate.
In practice, the curable coating composition of the present invention will be applied on a metal substrate. Metal substrates used in the practice of the present invention include ferrous metals, non-ferrous metals and combinations thereof. Suitable ferrous metals include iron, steel, and alloys thereof. Nonlimiting examples of useful steel materials include cold rolled steel, galvanized (zinc coated) steel, electrogalvanized steel, stainless steel, pickled steel, zinc-iron alloy such as Galvanneal, Galvalume and Galfan zinc-aluminum alloys and combinations thereof. Useful non-ferrous metals include aluminum, zinc, magnesium and alloys thereof. Combinations or composites of ferrous and non-ferrous metals can also be used.
At application, the temperature of the coating composition is typically about 10xc2x0 C. to about 85xc2x0 C., and preferably about 15xc2x0 C. to about 60xc2x0 C. For aqueous coating compositions, the pH of the coating composition at application generally ranges from about 7.0 to about 12.0, and is preferably about 8.0 to about 10.5.
If the pH of the coating composition needs to be adjusted, water-soluble or water-dispersible acids and/or bases can be used. Suitable acids include mineral acids, such as hydrofluoric acid, fluoroboric acid, phosphoric acid, and nitric acid; organic acids, such as lactic acid, acetic acid, hydroxyacetic acid, citric acid, and mixtures thereof. Suitable bases include inorganic bases, such as sodium hydroxide and potassium hydroxide; nitrogen-containing compounds such as ammonia, triethylamine, methyl ethanol amine, diisopropanolamine; and mixtures thereof.
The curable coating composition of the invention can be applied to the surface of a metal substrate by any conventional application technique, such as spraying, immersion or roll coating in a batch or continuous process. Squeegee or wringer rolls can be used to remove excess coating. After application, the curable coating is cured to form a cured coating upon the metal substrate. Curing can be achieved at peak metal temperatures of 100-400xc2x0 C. Peak metal temperatures of about 150xc2x0 C. to about 300xc2x0 C. are preferred. The cure times utilized in the present invention range from twenty (20) seconds to sixty (60) minutes.
The thickness of the applied coating is determined mainly by the application conditions. Generally, to achieve sufficient corrosion resistance for automotive use, the applied coating should have a film thickness of at least about 1 micrometer (about 0.04 mils), preferably about 1 to about 20 micrometers, and more preferably about 2 to about 10 micrometers. For other substrates and other applications, thinner or thicker coatings can be used.
One of the major advantages of the curable coating composition of the invention is that it can be applied either at a steel mill or a coil coating facility. When the coating composition is applied at a steel mill, the following steps are followed. First, the curable coating composition is applied-directly to a metal sheet shortly after it is formed and at a temperature of 20 to 150xc2x0 C. Second, the coating composition is dried using an IR oven. IR ovens generate the high peak metal temperatures in short periods of time (2 to 30 seconds).
When the coating composition is applied at a coil coating facility, the process is as follows. A metal sheet is unwound from a metal coil and passed through a cleaning station, a coating station, and a curing station in a substantially continuous manner. As the metal sheet passes through the coating station, the curable coating composition of the present invention is applied to the metal sheet. The coating composition is cured as the coated metal sheet passes through the curing station.
The present invention will now be illustrated by the following specific, non-limiting examples. All parts and percentages are by weight unless otherwise indicated.